Valve selection principles
Principles for selecting valves for water supply pipelines
① When the pipe diameter is less than 50mm, a stop valve should be used; when the pipe diameter is greater than 50mm, a gate valve or butterfly valve should be used.
② When it is necessary to adjust the flow rate and water pressure, the regulating valve and stop valve should be used
③ For parts with low flow resistance (such as the suction pipe of a water pump), gate valves should be used.
④ On pipe sections that require two-way flow, gate valves and butterfly valves should be used, and stop valves should not be used.
⑤ Where the installation space is small, butterfly valves and ball valves should be used
⑥ Stop valves should be used on pipe sections that are frequently opened and closed.
Selection of check valve
Check valves should generally be selected based on the following factors: installation location, water pressure in front of the check valve, air tightness performance requirements after closing, and size of water hammer caused by closing:
① If the water pressure in front of the valve is small, swing valves, ball valves and reciprocating valves should be selected.
② When air tightness after closing is strictly required, a check valve with a closing spring should be selected.
③ When it is necessary to weaken the cut-off water hammer, a quick-closing silencer check valve or a slow-closing check valve with a damping device should be selected.
④ The disc or valve core of the check valve should be able to close automatically under the action of gravity or spring force.
Set up check valves for water supply pipes
On the water inlet pipe; on the water inlet pipe of a closed water heater or tap water equipment; on the outlet pipe of the water pump; on the outlet pipe part of the water tank, water tower and highland pool, the inlet pipe and the outlet pipe are combined.
Note: It is not necessary to install a check valve on a section of pipe equipped with a pipe backflow preventer.
Location of water supply pipe exhaust device
① For water supply pipe networks that are used intermittently, the ends and highest points of the pipe network should be equipped with automatic exhaust valves
② The water supply pipe network has obvious undulating pipe sections where air accumulates. An automatic exhaust valve or manual valve is installed at the highest point of the section for exhaust.
③Pneumatic water supply device. When using an automatic air supply pressure water tank, the highest point of the water distribution pipe network should be equipped with an automatic exhaust valve.
Advantages and disadvantages of various types of valves
Gate valve
A gate valve refers to a valve whose closing member (gate) moves vertically along the axis of the channel. It is mainly used as a closing medium in pipelines, that is, fully open or fully closed. Generally, gate valves cannot be used to regulate flow. It can be applied to low temperature and high pressure as well as high temperature and high pressure, and can be based on different materials of the valve. But gate valves are usually not used in pipelines transporting mud and other media.
Advantages of gate valves:
① Small fluid resistance;
②The torque required for opening and closing is small;
③Can be used on ring network pipelines where the medium flows in both directions, that is, the flow direction of the medium is not restricted;
④When fully open, the corrosion of the sealing surface by the working medium is less than that of the stop valve;
⑤The body structure is relatively simple and the manufacturing process is better;
⑥The structure length is relatively short.
Disadvantages of gate valves:
①The overall size and opening height are large, and the space required for installation is also large;
②During the opening and closing process, the sealing surfaces are relatively frictional and abrasive, which can easily cause wear even at high temperatures;
③ Gate valves generally have two sealing surfaces, which makes processing, grinding and maintenance more difficult.
④Long opening and closing time.
Butterfly valve
Butterfly valves are valves that use disc-shaped opening and closing members to reciprocate at approximately 90° to open, close, and regulate fluid passages.
Advantages of butterfly valve:
① Simple structure, small size, light weight, saving consumables, not used for large diameter valves;
② Rapid opening and closing, low flow resistance;
③ Depending on the strength of the sealing surface, it can be used not only for media in which solid particles are suspended, but also for powdery and granular media. It is suitable for two-way opening, closing and adjustment of ventilation and dust removal pipelines, and is widely used in gas and water pipelines in metallurgy, light industry, electric power, and petrochemical systems.
Disadvantages of butterfly valve:
① The flow adjustment range is not large. When the opening reaches 30%, the flow rate enters more than 95%;
② Due to the limitations of the structure and sealing material of the butterfly valve, it is not suitable for use in high temperature and high pressure pipeline systems. The general working temperature is below 300℃ and below PN40;
③ The sealing performance is worse than that of ball valves and globe valves, so it is used in places where sealing requirements are not very high.
Ball valve
Ball valves evolved from plug valves. Its opening and closing part is a sphere, which is used to rotate 90° around the axis of the valve stem to achieve the purpose of opening and closing. Ball valves are mainly used to cut off, distribute and change the flow direction of media in pipelines. Ball valves designed with V-shaped openings also have good flow regulation capabilities.
Advantages of ball valves:
① Has the lowest flow resistance (actually 0);
② Since it will not get stuck when working (without lubricant), it can be reliably used in corrosive media and low-boiling point liquids;
③ Can be completely sealed within a large pressure and temperature range;
④ Rapid opening and closing can be achieved, and the opening and closing time of some structures is only 0.05~0.1s to ensure that it can be used in the automated system of the test bench. No vibration during operation when opening and closing the valve quickly;
⑤ The spherical cover can be automatically positioned at the boundary position;
⑥ Reliable sealing on both sides of the working medium;
When fully open and fully closed, the sealing surface of the ball and valve seat is isolated from the medium, so the medium passing through the valve at high speed will not cause corrosion of the sealing surface;
⑦ Compact structure and light weight, it can be considered the most reasonable valve structure for low-temperature medium systems;
⑧ The valve body is symmetrical, especially the welded valve body structure, which can withstand the stress of the pipeline well;
The closure can withstand high pressure differentials. Ball valves with fully welded bodies can be buried directly underground, so they will not corrode the valve’s internal parts and have a maximum service life of 30 years. It is the most ideal valve for oil and gas pipelines.
Disadvantages of ball valves:
① Since the main seat sealing ring of the ball valve is made of polytetrafluoroethylene, it is inert to almost all chemical substances, has a small friction coefficient, stable performance, is not easy to age, has a wide applicable temperature range, and has excellent sealing performance and other comprehensive characteristics. However, the physical properties of PTFE, including a high coefficient of expansion, sensitivity to cold flow and poor thermal conductivity, require that seat seals be designed with these properties in mind. Therefore, when the sealing material hardens, the reliability of the seal is compromised. Furthermore, PTFE has a lower temperature resistance rating and can only be used at temperatures below 180°C. Above this temperature, the sealing material will deteriorate. For long-term use, it can usually only be used at 120°C.
② Its adjustment performance is not as good as that of the stop valve, especially the pneumatic valve (or electric valve).
Globe valve
Refers to a valve in which the closing part (valve disc) moves along the centerline of the valve seat. According to this movement form of the valve disc, the change of the valve seat port is proportional to the valve disc stroke. Because the opening or closing stroke of the valve stem of this kind of valve is relatively short and has a very reliable closing function, and because the change of the valve seat port is proportional to the stroke of the valve disc, it is very suitable for flow regulation. Therefore, this type of valve is ideal for cutting off or regulating and throttling.
Advantages of globe valve:
① During the opening and closing process, the friction between the valve disc and the valve body sealing surface is smaller than that of the gate valve, so it is wear-resistant.
② The opening height is generally only 1/4 of the valve seat channel, so it is much smaller than the gate valve.
③ Usually there is only one sealing surface on the valve body and valve disc, so the manufacturing process is better and easy to maintain;
④ Since the filler is generally a mixture of asbestos and graphite, it has high temperature resistance. Typically, steam valves use stop valves.
Disadvantages of globe valve:
① Since the flow direction of the medium through the valve has changed, the minimum flow resistance of the stop valve is also higher than that of most other types of valves;
② Due to the longer stroke, the opening speed is slower than that of the ball valve.
